(1) Field of the Invention
The present invention relates to heater units and catalytic converters which can be suitably used for purification of automotive exhaust gases, etc.
(2) Prior Art
In addition to hitherto known porous ceramic honeycomb structures, metallic honeycomb structures have recently drawn attention as a catalyst, a catalyst carrier or the like for conversion of the nitrogen oxides (NO.sub.x), carbon monoxide (CO) and hydrocarbons (HC) contained in the exhaust gases discharged from the internal combustion engines of automobiles, etc.
Meanwhile, as the regulation for exhaust gases has become stricter, it is strongly desired to develop a heater or the like which can reduce emissions during the cold start of an engine.
U.S. Pat. No. 5,063,029 discloses a heater comprising a honeycomb structure with a resistance adjusting means (e.g. slits). U.S. Pat. No. 5,202,548 also discloses a method for holding a honeycomb heater in a casing by covering the periphery of the honeycomb heater with a metallic band via an insulating material (e.g. ceramic mat, ceramic cloth or the like).
In each of the above techniques, the resistance adjusting means is insulated and protected. Under severe driving conditions (vibration and thermal shock, in particular) of automobiles, however, the heater of U.S. Pat. No. 5,063,029 may develop detachment of the inorganic adhesive; and the heater of U.S. Pat. No. 5,202,548 may develop, owing to the vibration of horizontal and vertical directions, deformation of heater, breakage of spacer and wearing of insulating mat.
The present applicants made a further study and developed a heater unit which, when exposed to severe driving conditions of automobiles, etc. and subjected to vibration and the expansion and contraction caused by thermal shock, gives rise to neither breakage of the honeycomb heater nor peeling. (The heater unit was disclosed in SAE Technical Paper Series 940466.) This heater unit has substantially prevents deformation or breakage of honeycomb heater and is very preferable.
The heater unit has a structure comprising a honeycomb heater and a casing holding the honeycomb heater therein via a flexible metallic supporting member. In the SAE Technical Paper Series 940466, description is made also on a flexible electrode structure, a ring for gas flow restriction, and a light-off catalyst provided downstream of the honeycomb heater.